The inner ear contains, besides the three semicircular canals responding to angular acceleration, two so-called gravity receptors, the utricle and saccule, stimulated by rectilinear acceleration. Their function is essential for maintaining normal equilibrium. The thin, mobil otolithic membrane rests upon the ciliated sensory epithelium, providing mechanical stimulation to the sensory cells. The membrane is covered with a layer of calcite crystals called otoconia, which add weight and inertia. Virtually nothing is known about their metabolism. Findings in our study of human temporal bones and drug-treated animals suggest that otoconia are continuously being formed and are exposed to attrition and disturbed morphogenesis. The proposed study concerns the possible turnover of calcium in the otoconia. Radioactive calcium, Ca 45, will be given in repeated doses to various experimental animals: gerbils will be used initially. After animals have been sacrificed at different time intervals the entire otolithic membranes will be removed by means of microdissection. Samples will be weighed and counted by liquid scintillation spectrometry. This method is very sensitive and offers the best approach presently available. If there is an uptake of calicum we plan to quantitate the assimilation in relation to time and total dry weight of the otoconia. Furthermore, we propose to carry out autoradiography in an attempt to localize areas of activity. The effect of vestibulotixic drugs on the possible turnover of calcium will be studied by administering streptomycin and gentamicin in appropriate doses. In a final phase we plan to study the calicum uptake in animals with negative calcium balance (hyperparathyroidism, rickets, possibly immonibilization and senility) in order to find out how the otoconia relate to the body calcium pool. If no uptake is found in rodents we plan to study primates (squirrel monkey) and birds (pigeon). The study has basic importance for the clinical otologist. Since man now is exposed to extended periods of weightlessness and to the resulting disturbance of calcium metabolism, it should help to clarify some of the problems of space medicine.